Phytophthora capsici Leonian is an economically important soilborne pathogen of summer squash (Cucurbita pepo L.) and other vegetable crops in many areas of the world (Babadoost, 2004, 2005; Babadoost and Zitter, 2009; Hausbeck and Lamour, 2004; Hwang and Kim, 1995). Phytophthora capsici causes fruit, crown, and root rot as well as foliar blight (Babadoost, 2004, 2005; Babadoost and Zitter, 2009; Hausbeck and Lamour, 2004). Phytophthora crown rot is particularly severe because infections result in plant death and significant crop loss. Management of Phytophthora crown rot requires an integrated approach (Babadoost, 2005; Hausbeck and Lamour, 2004; Hwang and Kim, 1995; Ristaino and Johnston, 1999). Cultural management practices, including growing resistant cultivars, have been used to successfully manage other diseases caused by Phytophthora species including root rot of red raspberry (Rubus idaeus L.) caused by P. fragariae var. rubi (Wilcox et al., 1999), root rot of papaya (Carica papaya L.) caused by P. palmivora (Vawdrey et al., 2004), and leather rot of strawberry (Fragaria ×ananassa Duchesne) caused by P. cactorum (Madden and Ellis, 1990).
Host resistance is generally considered the most efficient means of controlling plant diseases. Resistance to P. capsici has been identified in pepper (Capsicum annuum L.) (Barksdale et al., 1984; Kimble and Grogan, 1960), and cultivars with resistance to Phytophthora crown rot are available. Unfortunately, few studies have screened unadapted cucurbit (Cucurbita spp.) germplasm for resistance to Phytophthora crown rot (Chavez et al., 2011; Padley et al., 2008). At least one C. pepo and five C. moschata germplasm accessions with resistance to Phytophthora crown rot were recently identified using P. capsici isolates from Florida (Chavez et al., 2011; Padley et al., 2008). Resistance to multiple P. capsici isolates was also identified in the Korean pumpkin cultivar Danmatmaetdol (C. maxima) (Lee et al., 2001). Resistance to P. capsici from the wild species Cucurbita lundelliana was introgressed into 19 winter squash breeding lines (Kabelka et al., 2007). An inheritance study indicated that resistance to Phytophthora crown rot derived from C. lundelliana and C. okeechobeenesis subsp. okeechobeenesis is conferred by three dominant genes (Padley et al., 2009). Resistance to P. capsici has not been incorporated into commercial breeding lines, and all cucurbit cultivars are considered susceptible to Phytophthora crown rot (Babadoost, 2004, 2005; Hausbeck and Lamour, 2004). However, cultivars may differ slightly in their reactions to P. capsici, and growing less susceptible cultivars in combination with other cultural management practices could be used to improve management of Phytophthora crown rot.
Cultural management practices that reduce soil saturation or prevent splash dispersal of P. capsici propagules can affect Phytophthora crown rot development. Growing plants on raised beds improves water drainage, thereby limiting the conditions favorable for disease development. Phytophthora blight incidence on pepper was 18% in flat beds and 5% in beds raised 45 cm (Hwang and Kim, 1995). Similarly, plant death of zucchini in a field naturally infested with P. capsici was greater in flat beds than raised beds (Hausbeck and Lamour, 2004). Covering planting beds with plastic mulch can reduce splash dispersal of P. capsici from the soil to susceptible plant tissues (Ristaino et al., 1997). However, in some cases, plastic mulches increased the spread of Phytophthora blight within a row because P. capsici propagules are readily dispersed in water on the surface of plastic mulches (Ristaino et al., 1997; Springer and Johnston, 1982). Organic mulches have also been effective at reducing splash dispersal of P. capsici. Chopped wheat straw dispersed between planting rows reduced the spread of Phytophthora blight on pepper (Ristaino et al., 1997). Altering cultural practices may not affect Phytophthora blight development on vining crops like watermelon, which grow off of raised beds and come into contact with the soil between rows (Kousik et al., 2011). Similarly, the costs associated with raised bed, plasticulture may not be feasible in crops grown for processing because crop values are lower.
Application of organic soil amendments such as composted animal manures has been used to suppress diseases caused by various soilborne pathogens including P. capsici (Zinati, 2005). Compost water extracts from livestock manures inhibited zoospore germination, germ tube formation, and mycelial growth of P. capsici (Sang et al., 2010). In a separate study, manure application reduced the viability of P. capsici oospores (Núñez-Zofío et al., 2011). Applications of compost water extracts increased the expression of numerous pathogenesis-related genes in pepper plants and reduced disease caused by P. capsici (Sang et al., 2010). Amending potting mix with composted sewage sludge reduced the incidence of Phytophthora crown rot on pepper by 42% in a greenhouse trial (Lumsden et al., 1983). Application of semicomposted horse and poultry manure followed by plastic mulching increased soil microbial activity and reduced the incidence of Phytophthora crown and root rot on pepper in Spain (Núñez-Zofío et al., 2011).
Combining cultural management practices, including planting cultivars with resistance to P. capsici, could be used to improve the management of Phytophthora crown rot on summer squash. Few large-scale field trials have evaluated the integrated use of multiple cultural practices to manage Phytopththora crown rot of summer squash. The objective of this field study was to evaluate the effects of bed height, mulches, composted poultry litter, and cultivars on Phytophthora crown rot of summer squash in the absence of fungicides. Thirty-two summer squash cultivars and 10 germplasm accessions were also evaluated for their reaction to Phytophthora crown rot in a separate greenhouse study.
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